The invention relates to an at least substantially metallic flat gasket comprising at least one first sheet metal layer consisting of a spring steel and at least one second sheet metal layer consisting of a ductile steel, the first layer being provided with at least one sealing bead, and the second layer being provided in at least one edge area bordering on a free, peripheral or an inner edge of the sheet metal layer with at least one thickened zone as supporting element for receiving pressing forces acting on the flat gasket when the latter is installed.
In particular, the invention relates to such a flat gasket as is designed as cylinder head gasket, and in the following the invention will be explained with reference to cylinder head gaskets, although a flat gasket according to the invention is also suitable for other applications, for example, for a gasket between a cylinder head and an exhaust manifold, or a so-called flange gasket between flanges of two pipes or other components that are to be joined to one another by screws.
Typical of such a flat gasket is, for example, the cylinder head gasket disclosed in US 2002/0063394-A1, which comprises two outer first sheet metal layers consisting of a spring steel and a second sheet metal layer arranged between these. Around the combustion chamber openings of this cylinder head gasket, the two outer first sheet metal layers are provided with sealing beads stamped into these. When installing the cylinder head gasket and while the engine is in operation, the sealing beads are protected against excessive flattenings owing to the pressing forces acting on the cylinder head gasket by the central, second sheet metal layer being provided around the combustion chamber openings with deformation delimiters (so-called stoppers), which are produced by a ring-shaped edge area of the second sheet metal layer having been respectively bent over and back onto the actual sheet metal layer around the combustion chamber openings, so that this edge area forms a ring-shaped, thickened zone. Although an incompressible, thickened supporting element is thereby obtained, this has a number of disadvantages: in the manner described, only a supporting element can be produced whose thickness is equal to twice the sheet thickness of the respective sheet metal layer, but a supporting element of greater thickness would be desirable for many applications. Furthermore, in the manner described, only a supporting element can be produced, whose width (i. e., its radial dimensions in relation to the associated combustion chamber opening) is relatively small, for if one wanted to produce a wider supporting element, the sheet metal would begin to tear from its free edge when folding over the said edge area.
In U.S. Pat. No. 1,587,626 a single-layered cylinder head gasket made from a sheet metal layer is disclosed, wherein the sheet metal layer has respectively been folded several times back onto itself around the combustion chamber openings in such a way that around each combustion chamber opening a thickened ring-shaped sealing element is produced, in whose cross section the sheet metal forms an S-shaped structure or a structure with two nested rings, whose cross section respectively corresponds to a lying U. Apart from this known cylinder head gasket being a single-layered gasket, not consisting of a sheet spring steel owing to the producibility of the sealing elements and, consequently, not being provided with the usual sealing beads that are elastic in the direction of their height, the thickened zones form sealing elements and not supporting elements to be added to sealing elements. Furthermore, in this known cylinder head gasket the thickness of a thickened zone must as a consequence always be a multiple of the sheet thickness, and, in addition, these thickened zones can only be produced with a slight width because the sheet metal would otherwise tear when being folded over.
In single-layered and multi-layered metallic cylinder head gaskets, thickened supporting elements are also used at other locations than around openings of the cylinder head gasket that are to be sealed: in a multi-cylinder engine the usual position of the cylinder head screws has as a consequence that the cylinder head in the area of its narrow sides, i. e., its longitudinal ends, has a tendency to be pulled downwards to a stronger extent, i. e., in the direction towards the engine block; in addition, the high gas pressures prevailing in the combustion chambers when the engine is in operation cause the cylinder head to arch albeit slightly between its longitudinal ends when the engine is in operation. These two effects have already been counteracted by providing the cylinder head gasket in the area of its longitudinal ends with thickened zones which serve as elevated supporting elements for the longitudinal ends of the cylinder head. To produce these supporting elements, tongue-like or lug-like edge areas provided at the periphery of a sheet metal layer of the cylinder head gasket are usually folded over and thereby placed back onto the actual sheet metal layer. In this case, too, the thickness of these supporting elements must as a consequence be equal to twice the thickness of the sheet metal. It is also problematic to produce at an edge area of the sheet metal layer, which in a plan view is round, a supporting element which extends from the edge of the cylinder head gasket over quite a large extent into the interior of the gasket because, as described hereinabove, the sheet metal then tends to tear when being folded over.
The object underlying the invention was to propose a flat gasket of the kind mentioned at the outset, where the design engineer is not governed by the limitations explained hereinabove when designing a supporting element.